In recent years, countermeasures, such as a reduction in a can production cost and an introduction of a new model of can, e.g., bottle cans and special shaped cans, on the market, have been instituted to arouse demand for steel cans.
Examples of measures for the reduction in can production cost include a reduction in material cost. Therefore, thickness reductions in steel sheets to be used have been pursued regarding not only two-piece cans associated with drawing, but also three-piece cans primarily associated with simple roll forming.
However, a simple thickness reduction in steel sheet causes a reduction in can body strength. Consequently, steel sheets having simply reduced thicknesses cannot be used for portions formed from high-strength materials, e.g., can body of Drawing-Redrawing Cans (DRD cans) and welded cans, and a very thin, high-strength steel sheet for a can has been required. At present, a very thin, hard steel sheet for a can is produced by a Double Reduce method (hereafter abbreviated as a DR method) in which secondary cold rolling is conducted after annealing. The steel sheet produced by using the DR method has a feature that the strength is high and the yield elongation is small. On the other hand, an application to cans, e.g., special shaped cans which have been introduced on the market recently, associated with can barrel forming, which is a high level of forming, is difficult because the DR material having low ductility exhibits poor formability. In addition, the cost becomes high because the steps for manufacturing the DR material increase as compared with common steel sheets produced by temper rolling after annealing.
To avoid the above-described drawbacks of the DR material, the following patents propose methods for manufacturing a high-strength steel sheet by a Single Reduce method (SR method) in which a secondary cold rolling is omitted and characteristics are controlled through a primary cold rolling step and an annealing step by using various enhancing methods.
Japanese Unexamined Patent Application Publication No. 2001-107186 proposes that a steel sheet for high-strength can on a DR level is produced by adding large amounts of C and N, followed by bake hardening. It is described that the yield stress after the lacquer baking treatment is a high 550 MPa or more, and the resulting hardness can be controlled by the amount of addition of N and a heat treatment.
Likewise, in Japanese Unexamined Patent Application Publication No. 11-199991, the strength is increased by about +50 MPa through the baking treatment after painting as in Japanese Unexamined Patent Application Publication No. 2001-107186.
Japanese Unexamined Patent Application Publication No. 8-325670 proposes a steel sheet keeping strength-ductility in balance by combining strengthening through precipitation of Nb carbides and strengthening through refining in grain size due to carbonitrides of Nb, Ti, and B.
Japanese Unexamined Patent Application Publication No. 2004-183074 proposes a method for increasing the strength by using strengthening through solid solution due to Mn, P, N, and the like.
Japanese Unexamined Patent Application Publication No. 2001-89828 proposes steel sheet for a can having a tensile strength of 540 MPa or less by using strengthening through precipitation of carbonitrides of Nb, Ti, and B and improved moldability of welled portion by controlling the particle diameters of oxide inclusions.
It is indispensable that the strength is ensured to achieve a thinner gauge. On the other hand, in the case where a steel sheet is used for a can body which undergoes a high level of can barrel forming, such as expand forming, or a can body which undergoes a high level of flange forming, it is necessary that a high-ductility steel is applied. Furthermore, a steel exhibiting small change in can height is required for expand forming.
In bottom forming of a two-piece can and can barrel forming typified by expand forming of a three-piece can, a strain at the same level as a few percent of tensile forming is provided. Consequently, it is necessary to apply a steel sheet having a small yield elongation to prevent generation of stretcher-strain. Furthermore, in consideration of the application to highly corrosive contents, a steel sheet exhibiting excellent corrosion resistance is required. Therefore, excessive addition of elements which impair the corrosion resistance is avoided.
In consideration of the above-described characteristics, a steel sheet which satisfies any one of the strength, the ductility, the yield elongation, and the corrosion resistance can be produced by the above-described known technologies. However, a steel sheet which satisfies all the properties cannot be produced.
For example, the methods described in Japanese Unexamined Patent Application Publication Nos. 2001-107186 and 11-199991 in which the strength is increased by adding large amounts of C and N, followed by bake hardening are methods effective for increasing the strength. However, since the amount of solute C and solute N is large, it is estimated that the yield elongation is large.
Japanese Unexamined Patent Application Publication No. 8-325670 describes that the strength is increased by strengthening through precipitation and proposes a steel keeping strength-ductility in balance at a high level. However, the yield elongation is not described. The yield elongation is not obtained by common manufacturing methods.
Japanese Unexamined Patent Application Publication No. 2004-183074 proposes the increase in strength by strengthening through solid solution. However, since P and Mn which are generally known as elements impairing the corrosion resistance are excessively added, there is a high probability that the corrosion resistance is impaired.
In Japanese Unexamined Patent Application Publication No. 2001-89828, a desired strength is obtained by using strengthening through precipitation of Nb, Ti, and the like and refining in grain size. However, from the viewpoint of the formability of a welded portion and the surface properties, addition of oxides of Ti, Ca, and REM is indispensable and, furthermore, it is necessary to control the particle diameters of the oxides. Therefore, an increase in cost and operation problems are expected.
It could therefore be helpful to provide a steel sheet for a can having such characteristics that after lacquer baking, the tensile strength becomes 450 to 550 MPa, the total elongation becomes 20% or more, and the yield elongation becomes 5% or less and exhibiting good corrosion resistance against highly corrosive contents and a method for manufacturing the same.